What 3D texture formats does OSL support?
OK, I've done it. In a sense.
The first and foremost thing to mention is that, as of the moment, Blender does not seem to support any voxel formats on an OSL level. I'm sure it will eventually, and the framework for texture3d seems to work fine; but there aren't any valid inputs. Someone please correct me if I'm wrong.
However, intrinsically, a voxel texture can be thought of as an array, along an additional axis, of pixel textures. I've successfully described a 3D shape by determining the z-height of the voxel I'm looking at, using format to create a file name of %03d.png, and looking the texture for that plane up in a directory. It runs very efficiently.

My specific code, so far, is as follows.
volume tex3d(float t = 0.0, point p = 0, int frame_count = 0, vector dimensions = 0, string path = &quot;&quot;, output float aOut = 0) {
    //align point to [0, 1] square
    point st = (p + vector(1.0))/2.0;
    
    //find frame by comparing against z-dimension of volume
    int frame = st[2] / (dimensions[2]/frame_count);
    
    //Get our file name, so we can pull a texture from the directory
    string sz = format(&quot;%03x.png&quot;, frame);
    aOut = 1.0 - texture(concat(path, sz), st[0], st[1]);
}

Ideally Blender will eventually support both VDB, as it already exports to it; and Field3D, which is an open sourced format by the same people who put OSL together. For now, this will do nicely.
Notably, the voxel cloud still does not match the blurb in the corner; this is because I drew it by hand. There's likely a way to convert VDBs to a sequence of PNGs, but I haven't done it as of yet.